Hot-stamping is a process whereby a transfer of material from a foil onto a thermoplastic surface is achieved by the application of heat and pressure to the foil and plastic. The transfer is generally in the form of a print, or copy, or a design, all of which is determined by the die effecting the transfer. In an actual deposition, the plastic is pressed against the die, sandwiching the foil between the plastic and die, effecting material transfer from the foil carrier to the plastic surface. When the transfer of material to the plastic surface occurs, it is absolutely necessary that the die effecting the transfer and the foil, be in contact with that part of the surface of the plastic which must be printed or decorated. Any lack of contact, no matter how small, will result in no deposit. Since the entire surface to be printed must then be in contact with the die, and under high pressure and high temperature to effect transfer, the surface of the plastic deforms due to a number of causes:
A. Relief of stresses and strains and, in some cases, memory; PA1 B. Change of state (melt); PA1 C. The introduction of laminar flow of the plastic at a multi-molecular depth from the surface. PA1 A. The "glassy" state, whereby the deformability of the plastic is so low that the curve virtually merges with the abscissa axis. This state exists at low temperature, and hot-stamping attempted at this end of the scale of the curve would require de-bossing of the plastic to effect a print of any sort. Since any deposit of the leaf requires some temperature, hot-stamping would probably take place at the glass-transition stage, designated T.sub.G. At this point deformations become reversible, and the hot-stamping print is vulnerable to cracking. PA1 B. The "rubbery" state is shown as the area with a long plateau on the curve. At these temperatures the polymer can develop high reversible deformations which reach their limit (under the given conditions of force action) at a distinct plateau. Hot-stamping simply could not be done successfully in this area. PA1 C. The viscoelastic (and/or viscofluid) state exists to the right of the rubbery state up to the thermal degradation of the polymer. The basic characteristic of this state is the development of indefinitely high irreversible deformations. It is in the transition temperature range (T.sub.f) where hot-stamping can be performed at maximum efficiency. PA1 A. True hot stamping is not possible in the glassy state because the energies of interaction are too large to allow chemical bonding, and the force which is required to work the surface so that the hot-stamping foil is fused into the surface is too large to be practical. PA1 B. Hot-stamping in the rubbery state is not plausible because the surface deformation is reversible, and mechanical surface bonding of the foil will be broken once the applied force has been removed. Chemical bonding is very unlikely because the energy of interaction of the macromolecules and their segments is too large. PA1 C. The viscoelastic state, where bonding energies are low, and the material flows, and deformation changes are irreversible, is the only region where true hot-stamping can take place, since chemical bonding can readily be achieved and the surface flow will allow the die to virtually act as a mold. PA1 A. the plastic composition; PA1 B. the extent (height) and depth of the surface flow; PA1 C. the velocity of movement of the plastic piece past the die (or vice versa). This would be the dwell time of the force applied perpendicularly to the plastic surface; PA1 D. friction, if any; PA1 E. the force due to the momentum of the impact of the plastic piece against the die; PA1 F. the thickness and composition of the piece being hot-stamped; and PA1 G. compliance and/or rigidity of the plastic. PA1 A. one in which a hot-stamping takes place only in the visco-elastic region of the thermomechanical curve and according to a system in which there can be infinitely adjusted temperature and pressure within limits to determine the appropriate region; PA1 B. The die is free to move in all directions of freedom and can be adjusted in all directions of freedom; PA1 C. The variable compliances are in the form of miniature air cylinders, or springs, so that compliance can be formed either by air pressure (or by mechanical spring force) with each cylinder adjusted by a needle valve. Air pressure in each cylinder is read on the gauge associated with each cylinder. However, these compliances may also be springs as noted, and they can be of various sizes and spring rates distributed as shown in the drawings; PA1 D. There are utilized means to convey the plastic piece past the die; this normally being in the form of a belt conveyor and it is imperative that the movement of the plastic be in a straight line at the point of passing the die and not in a radial path; PA1 E. On the conveyor belt or conveying means are fixed mandrels in which the plastic piece is held as it is conveyed past the die. In the case of containers where the mouth is smaller than the body, a special and unique tooling is usable on the conveyor or mandrel station. All these items, mandrel stations and mandrel mountings, are adjustable, and serve to allow hot-stamping at previously unattainable production rates; PA1 F. The present mechanism handles and advances the foil for hot-stamping operations, the foil being very thin and maintained at a certain prescribed tension to stamp properly. If the foil is not properly handled, it tends to wrinkle, scratch, or otherwise deform in a way which prevents a good print; PA1 G. Appropriate transducing and circuitry is provided to obtain force distribution appropriate for the hot-stamping process which allows determination of the characteristic "force matrix" by viewing the output of the transducers on an oscilloscope screen.
To understand the nature of the surface of molded plastic, it must be realized that when a plastic resin is molded (either by injection or compression, or blow-molded), one does not get a geometrically precise item. In the plastic surface, there are many hills and valleys which occur. These hills and valleys cause numerous problems for hot-stamping operations. Since contact is absolutely essential, it can be readily understood that various ripples and deformations in the plastic surface would prevent surface-to-surface contact. One way of overcoming this problem is to press the die against the plastic with such force that the surface to be printed is essentially equalled out to allow contact. What happens in this case, is that it can only be done safely at low temperatures, so that the types of deformation described previously do not occur; what does occur is, a de-bossing of the plastic surface takes place, not true, permanent, hot-stamping.
It is possible to hot-stamp by vertically pressing a heated die into the plastic, and this is called vertical, or flat, stamping. For using a curved surface (as, for example, a cylindrical bottle or lipstick tube) where more than 25 percent of the surface is to be stamped, then it is a common practice to roll the plastic on a mandrel past a die (flat or curved), and we call this peripheral hot-stamping. The present invention as described is applicable to both flat and peripheral stamping. It is, however, principally directed to peripheral stamping. The most significant difference, however, between the two forms of hot-stamping is that we deposit material over a broad area under a moderate force as quickly as a third of a second with flat stamping; we deposit material in the peripheral type stamping over a very small area under much greater force in, perhaps, 0.005 second, with the described peripheral stamping. We are theoretically printing on a line (a round surface tangentially contacting a flat surface).
These differences become significant when one considers the contribution of the leaf, or foil, to the phenomena of hot-stamping. Theoretically, when the foil material is deposited, it is not only mechanically pressed into the plastic, but should also combine chemically with the plastic being decorated. Thus, temperature, pressure, dwell time and the chemical nature of the leaf, are also much more critical when following a peripheral stamping, as opposed to the flat stamping. In the present invention, flat-stamping and peripheral stamping are both achievable because the adaption of the mechanism to the rheological considerations allows it to function identically in flat stamping as it does when applied to peripheral stamping, albeit peripheral stamping remains much more critical.